The present invention relates to a method of heating food and a food heating apparatus.
In conventional cooking apparatus or ovens utilizing humidity sensing, for example, known from U.S. Pat. No. 4,097,707 issued June 27, 1978, the time length .tau..sub.o from the start of heating the food to the boiling of the food is measured, and from the time point when the food begins to boil, the food is further heated for a time length which is obtained by multiplying the time .tau..sub.o by a specific constant K depending on the type of food and the desired finished condition until the food is completely cooked. The amount of heat required before the cooking of the food is finished as desired is considered to be expressed by the equation below. EQU Q=n.multidot.C.multidot.(Tv-Ti)+m.multidot.B (1)
where
C: specific heat of the food, PA1 m: quantity of the food, PA1 Tv: temperature at which the water in the food begins to boil, PA1 Ti: initial temperature of the food, and PA1 B: a value specific to the food or the value dependent on the amount of heat associated with the change in quality (or degeneration) and the latent heat of evaporation.
The value Q is given as the product of the microwave power p and the total heating time of the food .tau., namely, Q=p.multidot..tau.. Thus, the total heating time of the food is expressed as ##EQU1## The first term of the right side of this equation represents the time length from the starting of heating the food to the boiling of the water in the food, and the second term thereof is considered to represent the time from the start of boiling of the food to the properly cooked up state of the food. Therefore, the equation (2) may be rewritten as ##EQU2##
In the conventional cooking ovens using the detection of humidity, the initial temperature of the food Ti in equation (3) above is assumed to be about 20.degree. C. so that K is considered as a constant, with the result that the food heating time is controlled by equation (3).
The food to be heated for cooking, however, includes that which has been left to stand at a considerably high room temperature, that just taken out of the refrigerator, and frozen food, etc., which vary greatly from one another in the initial temperature immediately before being heated. The value K for such a wide range of food is not constant but greatly varied. In the prior art cooking ovens wherein the value K is assumed to be constant, food sometimes failed to be finished satisfactorily on account of the great variety of the initial temperature of the food to be cooked and the resulting wrong setting for the total heating time.
In a method of food heating control proposed to obviate this disadvantage, the food is heated slowly at low output or heating and suspension of heating of the food are alternated so that the internal temperature of the food is approximated to the surface temperature thereof, and under this condition, the time point when the surface temperature reaches a predetermined set value is detected by an infrared detector. From this time point of detection, the food is further heated for a time length obtained by multiplying the constant K specific to the food by a time length from the above-mentioned time point of detection to the time point of humidity detection when the humidity increases by violent generation of vapor, thus cooking the food.
The set value of the surface temperature is the initial temperature Ti in equations (1), (2) and (3) above. Thus the value K in equation (3) is also required to be determined anew by experiments. Assume, for instance, that the new initial temperature, namely, the set value is 50.degree. C., the heating time from the start of heating to the time point of violent generation of vapor from the food when the humidity is detected is .tau..sub.o ', and the heating time from humidity detection time to the time point when the food is cooked up properly is .tau..sub.h '. Then the new constant K is given as .tau..sub.h '/.tau..sub.o ' as in the case of equation (3). In this method, the set value of initial temperature is required to be varied with food and determined by experiments, thereby complicating the cooking procedures.
Another shortcoming of this method is that the fact that the food temperature has reached the set value is detected by reference to the surface temperature thereof, and therefore in order to avoid a great error between the detected surface temperature and the internal temperature of the food being heated, the food is required to be heated sufficiently slowly or the heating and suspension of heating are required to be alternated thereby to attain the internal temperature as near to the surface temperature as possible, resulting in an excessively long cooking time.
Another proposed heating control method is such that the initial temperature of the food is measured and the food is heated first to a predetermined set value of the surface temperature thereof. The food is then further heated for a time length obtained by multiplying the heating time required for the food surface temperature to reach the set value from the start of heating, by a multiplicator calculated from the food initial temperature, the set value, a factor specific to the food to be cooked, the specific heat of the food and the microwave absorption rate of the food, thus cooking the food. This method also has the disadvantage that unless the food is heated slowly up to the predetermined set value of the surface temperature, there occurs a great difference between the surface temperature and the internal temperature of the food, so that the surface temperature fails to represent the food temperature as a set value, and therefore the total heating time cannot be determined properly. As a result, it takes a long time to attain a surface temperature near to the internal temperature of the food.